iCloud Drive makes it possible to store your projects in the cloud. Any projects you add to the iCloud Drive location in Concepts will be synced across your devices.
iCloud Drive will appear as a location in the gallery sidebar above the local On This iPad location.
Any folders or drawings under iCloud Drive are stored in the Concepts folder in your iCloud Drive and can be accessed on all devices that are signed into that same iCloud account.
Any folders or drawings under On This iPad are stored locally on your device and can only be accessed from that specific device.
As Concepts is integrated with the iOS Files app, you can access all of your drawings there as well. Just navigate to Files > iCloud Drive > Concepts to find them. If you reorganize your drawings in Files, the Gallery in Concepts will update to use the same organization. This also works the other way around - all changes made in Concepts are mirrored in Files.